Three-dimensional integrated circuits (3DICs) include a plurality dies stacked atop one another. In some approaches, a first wafer including a plurality of a first type of dies is bonded to a second wafer including plurality of a second type of dies. A functionality of the first type of die is different from a functionality of the second type of die. Each first type of die is aligned with a corresponding second type of die and electrical connections are formed between the first type of die and the second type of die by bonding the first wafer to the second wafer. The bonded wafers are then sawed and separated die stacks are usable with other components of a 3DIC.
A size of each first type of die closely matches a size of the corresponding second type of die, and the dies are bonded in a 1:1 ratio. Non-overlapping areas of the each of the first type of dies or the corresponding second type of die are filled with a fill materials, such as an underfill material, a molding compound, a dielectric material or another such material.